IC tests fall into a function test and a DC test. The function test is a test to see if the IC under test performs predetermined functions. The DC test is a test to see if a leakage current at an input terminal of the IC under test, for instance, is smaller than a predetermined value, or if the output current at an output terminal is larger than a predetermined value.
The present invention is directed to improvements in an applied-voltage-based current measuring device for use in the DC test; in particular, the invention is to adapt the current measuring device to be capable of measuring current values accurately without using any highly precision resistors and at high speed without switching between current measuring ranges.
In FIG. 4 there is depicted an example of a conventional applied-voltage-based current measuring device. In FIG. 4, reference numeral 11 denotes an IC under test, 12 a voltage source from which a predetermined voltage is applied to a terminal of the IC under test, and 13 current measuring means for measuring current that flows to the terminal of the IC under test 11 during periods of voltage application. The applied-voltage-based current measuring device is composed of the voltage source 12 and the current measuring means 13.
The voltage source 12 is made up of an operational amplifier 12A and a D/A converter 12B which supplies a voltage equal to a voltage to be applied to a terminal of the IC under test 11. To an inverting input terminal of the operational amplifier 12A is fed back from a sensing point (a voltage sensing point) SEN a voltage V.sub.1 applied to the terminal of the IC under test 11; by this feedback operation, the voltage V1 is made to match a voltage V.sub.DA from the D/A converter 12B, thus applying an intended voltage (for example, a voltage that defines H logic and L logic) to the terminal of the IC under test 11.
The current measuring means 13 is made up of a current measuring resistor R1 connected between an output terminal of the operational amplifier 12A and the sensing point SEN, a subtractor circuit 13A for extracting a voltage that develops across the current measuring resistor R1, and an A/D converter 13B for A/D conversion of the voltage extracted by the subtractor circuit 13A.
Incidentally, reference characters R2 and R3 denote resistors for range changeover use. These range change-over resistors R2 and R3 are adapted to be connected in parallel to the current measuring resistor R1 through selective turning-ON of range change-over switches S2 and S3, thereby switching the current measuring means 13 between its current measuring ranges.
That is, when the range change-over switches S2 and S3 are both in the OFF state and when the range change-over switch S2 is in the ON state, the current measuring means is put in the state of measuring a leakage current at one of terminals of the IC under test which is held in the input mode, and it measures minute currents approximately in the range of several to tens of microamperes. To perform this, the resistance values of the current measuring resistors R1 and R2 are chosen relatively large on the order of tens of kiloohms.
On the other hand, the resistance value of the range switch-over resistor R3 is chosen relatively small, for example, 10 ohms or so. Accordingly, when the switch S3 is put in the ON state, the current measuring means is switched to a measuring range of a relatively large current value, and measures current is provided from that one of terminals of the IC under test 11 which is held in the output mode.
The circuit configuration of FIG. 4 has the defect of requiring the range change-over switches S2 and S3. In other words, since the range change-over switches S2 and S3 are connected in series to the circuit through which the current to be measured flows, it is necessary, in particular, to minimize the ON-state resistance of the range change-over switch S3 through which a large current flows. On this account, a CMOS-structured semiconductor switching element cannot be used as the range change-over switch S3, but instead a relay is used commonly. Because of the relay's slow response, much time is needed for switching the measuring range. Furthermore, the current measurement starts at a high-sensitivity measuring range, and when the measured value falls outside it, the measuring range is switched to the next one and the current measurement is carried out again. This inevitably provides disadvantages that the switching of the measuring range is time-consuming and that when the measuring range is switched from a minute to a large current one (by turning ON the switch S3), much time is also taken for the circuit to settle back after the switching of the measuring range. The prior art example has another disadvantage of requiring much time for test because of repeating for each terminal the measurements of a leakage current and the output current of the IC under test 11 through the use of a single applied-voltage-based current measuring device.
Besides, the subtractor circuit 13A comprises, as depicted in FIG. 4, an operational amplifier A1, a buffer amplifier A2 and resistors R11, R12, R13 and R14. In this arrangement the resistance values of the resistors R11, R12, R13 and R14 need to be set, for example, such that R12/R11=R14/R13=1. This relationship of the resistors R11 to R14 has a significant effect on the determination of the gain of the operational amplifier A1 and the determination of its common-mode rejection ratio. Since the resistance values of the resistors R11 and R12 to R14 must therefore be set with high accuracy, the manufacturing cost of the subtractor circuit 13A is high, in particular, when implemented in an IC, because of difficulty in setting the resistance values of the resistors R11 to R14 with high accuracy.
As a solution to the above problems, there has been proposed such a circuit as shown in FIG. 5. In this circuit a series circuit of the range change-over resistor R2 and the range change-over switch S2 and diodes D1 and D2 are connected in parallel to the current measuring resistor R1, and the resistor R3 for large current measuring use is independently connected in series to the current measuring resistor R1; voltages that develop across the resistors R1 and R3 are extracted by subtractor circuits 13A and 13A', and the voltages thus extracted by the subtractor circuits 13A and 13A' are selectively provided via switches S22 and S33 into the A/D converter 13B.
With this circuit configuration, in a minute current region (a current region in which the diodes D1 and D2 remain OFF) the switch S22 is held in the ON state, through which a voltage developed by a current flow through the current measuring resistor R1 or a parallel circuit of the resistors R1 and R2 is applied to the A/D converter 13B to measure a minute current (a leakage current at each terminal of the IC under test 11).
In a large current region, since the voltage across the current measuring resistor R1 exceeds a value at which the diode D1 or D2 turns ON, a large current bypasses R1 through the diode D1 or D2 and the voltage across the current measuring resistor R1 is clamped at a conduction voltage (for example, 0.6 V or so) of the diode D1 or D2; in this state, a voltage across the current measuring resistor R3 is extracted by the subtractor circuit 13A' and provided via the switch S23 to the A/D converter 13B for measuring the large current.
With the circuit configuration depicted in FIG. 5, it is only the switch S2 that is used as the range change-over switch. Since the current measuring range is not greatly changed by the operation of the range change-over switch S2, the settling time of the circuit is short. In addition, since the circuit is a minute current circuit, it is possible to use a DMOS type semiconductor switch as the range change-over switch S2 and hence permit reduction of the change-over time as well. This provides an advantage that the time for switching the measuring range can be reduced.
However, this prior art example requires two subtractor circuits as identified by 13A and 13A', which makes its manufacturing costs higher than in the case of FIG. 4. Moreover, the use of the DMOS-structured switching element as the range change-over switch S2 would provide the disadvantage of further raising the overall manufacturing costs since the DMOS-structured switching element is expensive.
An object of the present invention is to provide an applied-voltage-based current measuring method which permits high-speed measurements of minute to large currents and an applied-voltage-based current measuring apparatus for implementing the method at low cost.